Large volume preconcentration and purification for determining the240Pu/²³⁹Pu isotopic ratio and ²³⁸Pu/^239+240Pu alpha-activity ratio in seawater

Summary

The present paper describes a new analytical method for determining the ²⁴⁰Pu/²³⁹Pu isotopic ratio and ²³⁸Pu/^239+240Pu α -activity ratio in seawater, both of which are important parameters for determining Pu sources in the ocean. Plutonium isotopes were preconcentrated from a large volume of seawater (4700-10800 liter) by solid phase extraction using MnO₂-impregnated fibers and eluted into 3M HCl. After the elution, the Pu species of all oxidation states were converted to Pu(IV) using NaNO₂, purified by solvent extraction using thenoyltrifluoroacetone (TTA)-benzene, and concentrated in 5 ml of 0.2M HNO₂. The ²⁴⁰Pu/²³⁹Pu and ²³⁸Pu/^239+240Pu ratios in the 5-ml final solution were determined by inductively coupled plasma-mass spectrometry (ICP-MS) and α-spectrometry, respectively. A pg level of Pu, which was a sufficiently large amount for the determination, was obtained by the solid phase extraction. Through the redox conversion and solvent extraction, the Pu species, such as Pu(III), Pu(IV) and Pu(VI), were collected at a high recovery of 96±2% (n=3) despite the presence of large amounts of Mn, and interfering ²³⁸U (3.3 µg^. l^-1in seawater) was effectively removed with a decontamination factor of 1.7·10⁷. The accuracy of the method for the ²⁴⁰Pu/²³⁹Pu ratio was verified using reference materials of seawater and a terrestrial soil sample. The present technique was applied to the determination of the ²⁴⁰Pu/²³⁹Pu and ²³⁸Pu/^239+240Pu ratios in coastal and oceanic water.